Moisture measurement apparatus, system and method utilizing microwave or high frequency energy

ABSTRACT

Apparatus for measuring a characteristic of a body of material having first and second substantially parallel coaxial lines with distal extremities in relatively close proximity to each other and adapted to be disposed in close proximity to the body of material. A transmitter is coupled to the first coaxial line for introducing electrical energy in the high frequency to a microwave range into the first coaxial line to cause an electromagnetic field to be established at the distal extremity of the first coaxial line and to extend into the body of material. A receiver is coupled to the second coaxial line for detecting any of the electromagnetic field coupled into it from the first coaxial line. A measurement device is coupled to the receiver means for ascertaining when any change occurs in the coupling of the electromagnetic field from the first coaxial line to the second coaxial line to determine said characteristic of the body of material.

This invention relates to a moisture measurement apparatus, system andmethod utilizing microwave or high frequency energy and moreparticularly to such a moisture measurement apparatus to and apparatus,system and method for ascertaining the glue applied to corrugatedcardboard.

Moisture measurements have heretofore been made in materials byutilizing capacitance type measurements or infrared sensing devices orlarge area microwave reflection and absorption devices. In themanufacture of corrugated cardboard, it has been found that it isdifficult during manufacture of the same to ascertain whether the properamount of glue has been applied between the internal corrugated sheetand the outer sheets of paper generally called liners. The integrity ofthese glue joints is critical to the quality of the end productcorrugated cardboard in terms of strength and flatness. There istherefore a need for an apparatus, system and method for resolving thesedifficulties.

In general, it is an object of the present invention to provideapparatus, system and method which utilizes microwave or high frequencyenergy in non-destructive testing to detect a physical characteristicsuch as moisture in a body of material.

Another object of the present invention is to provide an apparatus,system and method of the above character in which a highly localizedelectromagnetic field is utilized.

Another object of the present invention is to provide an apparatus,system and method of the above character in which the disturbances ofthe electric field are measured to ascertain the physicalcharacteristic.

Another object of the invention is to provide an apparatus, system andmethod which can be utilized when relative movement is occurring betweenthe sensor and the body of material.

Another object of the invention is to provide an apparatus, system andmethod of the above character which can be utilized for ascertaining thepresence or absence of glue at the joint between a corrugated sheet anda liner by measuring the moisture content of the glue.

Another object of the invention is to provide an apparatus, system andmethod of the above character in which fine spacial resolution can beobtained.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiments are set forthin detail in conjunction with the accompanying drawings.

FIG. 1 is a schematic illustration of apparatus and system utilizingmicrowave or high frequency energy incorporating the present inventionfor ascertaining the presence or absence of glue in the joints betweencorrugations and a liner of corrugated cardboard.

FIG. 2 is a side elevational view of a sensor utilized in the apparatusshown in FIG. 1.

FIG. 3 is a cross sectional view taken along the line 3--3 of FIG. 2.

FIG. 4 is a bottom plan view taken along the line 4--4 of FIG. 2.

FIG. 5 is a cross sectional view of the coaxial transmission line shownin FIG. 2.

FIG. 6 is a block diagram of the electronic instrumentation used withthe sensor shown in FIG. 2 and which is utilized in the apparatus andsystem shown in FIG. 1.

FIG. 7A is a schematic illustration showing the highly localized highfrequency electric field which is generated at the distal extremities ofthe contact head shown in FIG. 2 when in close proximity to or incontact with a poorly conducting body of material and the low level ofcoupling between the transmitting and receiving coaxial lines.

FIG. 7B is a schematic illustration similar to FIG. 7A but showing themanner in which enhanced coupling is provided between the transmittingand receiving coaxial lines when a high dielectric constant body ofmaterial is present at the contact head as for example a watercontaining compound.

FIG. 8 is a cross sectional view of corrugated cardboard produced inconnection with the present invention.

FIG. 9 is a graph showing the manner in which the apparatus and systemshown in FIG. 1 can be utilized to ascertain the glue distributionbetween the corrugations and the top liner of the corrugated cardboardin accordance with the present invention.

In general, the apparatus for measuring a physical characteristic of abody of material consists of first and second substantially parallelcoaxial lines having distal extremities adapted to be disposed in closeproximity to each other and to the body of material. Transmitter meansis coupled to the first coaxial line for introducing electrical energyin the high frequency or microwave range into the first coaxial line tocause an electromagnetic field to be established at the distal extremityof the first coaxial line and in the body of material. Receiving meansis coupled to the second coaxial line to detect any of theelectromagnetic field produced by the first coaxial line which iscoupled into the distal extremity of the second coaxial line.Measurement means is coupled to the receiving means for ascertaining theenhancement of the coupling of the electromagnetic field between thedistal extremities caused by the presence of moisture in the body ofmaterial to thereby ascertain the moisture content in the body ofmaterial.

More particularly as shown in FIG. 1, there is provided an apparatus andsystem 11 which is utilized for producing sheets 12 of corrugatedcardboard in which a corrugated sheet 13 is disposed between upper andlower sheets or liners 14 and 16 (see FIG. 8). The corrugated sheet 13is provided with a form which is similar to a sine wave and is providedwith peaks 17 which engage the inner surfaces of the liners 14 and 16and adhered thereto by glue 18 to form glue joints between the peaks andthe liners. The valleys between the peaks 17 are identified as flutes 19with the flute pitch being approximately equal to the spacing of thecorrugations.

The apparatus 11 consists of first, second and third rolls which can becharacterized as a bottom roll 21, a middle roll 22 and a top roll 23which carry rolls of the paper to be utilized to make the corrugatedcardboard 12. The sheet 26 applied from the bottom roll 21 passesthrough a glue applicator 27 which is provided with a roller 28 thattravels through a heated glue pan 29 and picks up glue therefrom andtransfers it to another roller 31 which applies glue to peaks 17 of thecorrugated sheet 13. This sheet 13 formed from a sheet 32 passing fromthe middle roller 22 and passing over an idler 33 and between a pair ofcorrugated rollers 34 and 36. The corrugated sheet 13 with the groovesthereon is brought into contact with the upper surface of the sheet 26and is glued thereto so that the sheet 26 forms the outer or bottomliner 14. The corrugated sheet 13 and the outer liner 14 then travelthrough a second glue applicator 37. The glue applicator 37 is similarto the glue applicator 27 and is comprised of a roller 38 which travelsin a heated glue pan 39 to pick up glue and to transfer the same toanother roller 41 which transfers glue to the peaks 17 of the other sideof the corrugated sheet 13. As soon as the corrugated sheet 13 passesfrom the glue applicator 37, it is brought into contact with anothersheet 46 which is payed off from the top reel 23 and passes over idlerroller 47 and under another idler 48 which brings the sheet into contactwith the upper glued peaks 17 of the corrugated sheet 13 to provide thetop line 16. The portion of the apparatus 11 heretofore described issubstantially conventional.

In accordance with the present invention, a sensor 51 is provided whichis adapted to engage the top liner 16 of the corrugated cardboard 14.The sensor 51 is connected to sensor instrumentation 52. The output ofthe sensor instrumentation 52 is coupled to a control console 53 of aconventional type which utilizes information being sensed by the sensor51 for controlling the glue pickup roll 38 to control the amount of gluewhich is picked up and transferred to the roller 41 and turn to thepeaks of the corrugated sheet 13.

The sensor or sensor apparatus 51 in FIG. 1 is shown in more detail inFIG. 2 and as shown therein is comprised of first and second coaxiallines or tubes 61 and 62 which are capable of carrying high frequencyand/or microwave energy. The coaxial lines 61 and 62 have distalextremities 63 and 64 which are carried by a contact or probe head 66.The coaxial lines 61 and 62 run generally parallel to each other and arebrought into close contact to each other with their distal extremitiesbeing cut off abruptly at the lower extremity of the contact head 66.

The contact head assembly 66 is comprised of two usually molded parts66a and 66b with outwardly open slots 67 and 68 provided on oppositesides of the same which are adapted to be brought into registration witheach other. Each part is also provided with semi-circular slots 69, and71 extending vertically of the parts and opening outwardly through theinnermating surfaces of the parts 66a and 66b to provide verticalchannels for the coaxial lines 61 and 62. The two parts 66a and 66b arefastened together in a suitable manner such as by a pair of screws 72extending through the slots 67 and 68 and threaded into support block73. The support block 73 is carried by the distal extremity of a springarm or plate 74 formed of a suitable spring material as for examplespring steel or phosphor bronze. As hereinafter described the spring arm74 is positioned to apply a light yieldable force to maintain thecontact assembly 66 in engagement with the surface of the material onwhich measurements are to be made, as for example the presence orabsence of glue lines in corrugated cardboard 12. The distal extremityof the spring arm 74 is secured to the support block 73 by a suitablemanner such as by screws 76. By way of example, the spring arm 74 couldhave a length ranging form 6-8" and a width ranging from 1" and asuitable thickness as for example 0.02'. In order to minimize themovement between the spring arm 74 and the coaxial lines 61 and 62, thecoaxial lines are bonded to the sides of the spring arm 74 as shown inparticular in FIG. 3 in a suitable manner such as by an adhesive.

The parts 66a and 66b of the contact head assembly 66 are formed of anabrasion resistant material which also is thermally and electricallyconductive. One material found particularly suitable for these parts 66aand 66b is Lanxide (trademark) manufactured by Lanxide ElectronicsComponents, LP., located at 1300 Marrows Road, Newark, Del., 19714-6077.The material is identified as MCX622 and is comprised of an aluminumsaturated matrix of silicon carbide crystals. The aluminum in theLanxide material provides structural strength while at the same timeproviding high thermal conductivity and good electrical conductivity.The silicon carbide content of the material provides outstandingabrasion resistance so that in combination the material provides athermal conductivity and an electrical conductivity almost as good aspure aluminum and abrasion resistance almost as good as that of siliconcarbide. The data supplied on the Lanxide material shows that it has anabrasion test volume loss substantially less than that of stainlesssteel. This is a very desirable characteristic for the contact headassembly 66 so that it will have a long life in industrial applicationmaking possible infrequent changing of the contact head assembly 66.Since the material for the parts 66a and 66b is very hard, it must bemolded into the desired shape as for example the shapes described above.The Lanxide material used for the parts 66a and 66b also has anadditional advantage in that it is very light as for example a specificgravity similar to that of aluminum. This is desirable to provide a lowmass at the distal extremity of the spring arm or plate 74 to therebymake it possible for the contact head assembly 66 to be maintained inintimate contact with the body of material being probed and permitting arapid relative movement between the contact head assembly 66 and thebody of material without skipping or jumping when undulations occur inthe body material.

The lower surfaces of the parts 66a and 66b are arcuate and rounded sothat when the two parts 66a and 66b are mated, semi-cylindricalsurfaces, as for example 1/2" in radius, are provided so that the headassembly 66 makes contact with the surface to be contacted along animaginary line which extends through the two extremities of the coaxiallines 61 and 62. A suitable yieldable force applying the contact headassembly to the material being probed was found to range from 5 to 20grams and preferably approximately 10 grams.

In considering possible substitutes for the Lanxide material, materialshaving a thermal conductivity degraded by a factor of two over thatprovided by aluminum and having an electrical conductivity degraded by afactor of 5 with respect to aluminum could be utilized. With respect toabrasion resistance, degradation of abrasion resistance can be tolerateddepending upon the application and the degree to which replacement ofthe contact heads is economically feasible.

The proximal extremity of the spring arm 74 is secured to a right anglebracket 81 by suitable means such as screws 82. The bracket 81 issecured to a mounting bar 83 by suitable means such as screws 84extending through vertically extending slots 86 provided in the bracket81 permitting the bracket 81 to be adjusted vertically of the bar 83.This makes it possible to adjust the spring force which is applied bythe spring arm 74 to the contact head assembly 66 to maintain it inlight engagement with the surface of the material to be contacted and onwhich measurements are to be made. As shown particularly in FIG. 4, thedistal extremities of the two coaxial lines 61 and 62 are flush with thesemi-circular contact surface of the contact head assembly 66.

As is well known to those skilled in the art, the spring arm is designedkeeping in mind the dynamic calculations which of necessity mustconsider the mass of the head assembly 66, the moment inertia of thecoaxial lines 61 and 62, the moment of inertia of the spring arms 74.Also to be considered should be the vibration which is introduced intothe contact head 66 during relative movement between the head 66 and thematerial which is being probed to ensure that a negative G accelerationis not encountered. In other words, it is desirable that the spring arm74 provide approximately constant load to the contact head assembly 66over a relatively large deflection distance for the head assembly.

A cross sectional view of the coaxial line 61 is shown in FIG. 5. Theother coaxial line 62 is substantially identical. By way of example, thecoaxial lines 61 and 62 can have a characteristic 50 ohm impedance. Theyare provided with a center copper conductor 91 with a suitable diameteras for example 0.025' which is surrounded by a solid cylindrical spacer92 of a suitable insulating or dielectric material such a Teflon(trademark) and of a suitable outside diameter as for example 0.085'.The dielectric spacer 92 is covered by a jacket 93 serving as an outershield and formed of a suitable conducting material such a copper whichtypically can be either tin plated or silver plated.

In accordance with the present invention, it is desirable that thecoaxial lines 61 and 62 operate in a frequency range from approximately10 megahertz to 20 gigahertz and in the present invention at a frequencyof 915 megahertz which is the center of the United States spectral bandfor industrial, scientific and medical uses.

Although the present invention has been described in conjunction withcoaxial lines, it should be appreciated that if higher frequencies aredesired that circular, ridge, elliptical or rectangular waveguides canbe utilized for very shallow penetrations. This would permit operationup to as high as 140 gigahertz. Lower frequencies are utilized when itis desired to penetrate thicker materials. In chosing a frequency, it isalso desirable to ascertain the type of resolution desired in themeasurements being made.

The sensor instrumentation 52 is shown in more detail in FIG. 6. Asshown therein it consists of a high frequency microwave generator 101 ofa conventional type to generate power range from 0.1 of a milliwatt toapproximately one watt in the desired frequency. By way of example inconnection with the apparatus shown in FIG. 1, a frequency of 915megahertz was selected with a power output of 10 milliwatts. Thiselectrical energy was supplied to the coaxial line 61 to produce at thedistal extremity 63 a highly localized high frequency electromagneticfield as hereinafter described that passes into the body of materialengaged by the contact head 66 which as shown can be in the form ofcorrugated cardboard 12. Any of the electric field created at the distalextremity 63 and coupled to and sensed by the distal extremity 64 of thecoaxial line 62 is supplied to a high frequency or microwave detector102 which provides an analog or digital output 103 that is supplied to adisplay oscilloscope 104 to generate a waveform 106 depicting the sensedinformation. The same output 103 can be supplied through a line 107 to awaveform analyzer 108 which is provided with an output 109 supplied to asuitable printout device or meters 111 as for example a plotter. Thewaveform analyzer 108 is also provided with an output 112 that suppliescontrol signals to the control console 53 to control the apparatus inthe production line for the corrugated cardboard 12.

The operation and use of the apparatus and system 11 utilizing microwaveor high frequency energy for making moisture measurements in accordancewith the method of the present invention may now be briefly described asfollows. Let it be assumed that it is desired to ascertain the amount ofglue which is being applied to the peaks 17 of the corrugated sheet 13which is to be utilized to form joints between the peaks 17 and theinside surface of the top liner 16. The sensor instrumentation 52supplies high frequency or microwave energy through the coaxial line 61which creates a highly localized high frequency electromagnetic fieldrepresented by the electric field lines 116 in FIG. 7A which passthrough the top liner 16, a poorly conducting medium.

The electric field is in effect a leakage or fringe radiation field fromthe distal extremity of the transmission coaxial line 61. A very smallamount of this electric field is coupled into the nearby similarlyinterrupted receiving coaxial line 62 as represented by the electricfield line 117 in FIG. 7A. In connection with the present invention, ithas been found that this highly localized field created at the distalextremity of the coaxial transmission line 61 is extremely sensitive toany dielectric material which is placed within its zone of majorinfluence which is formed at the two terminated coaxial lines 61 and 62to thereby provide a transducer for making dielectric measurements.

As shown in FIG. 7B, when such a transducer comprised of the coaxiallines 61 and 62 probe a body of material having a high dielectricconstant and disposed in the zone of major influence of the leakageradiation field, there is provided a greatly enhanced coupling betweenthe transmitting and receiving coaxial lines 61 and 62 represented bythe plurality of electric field lines 117. This greatly enhancedcoupling is immediately recognized by the detector 102 which supplies awaveform 106 such as shown in FIG. 9 which has peaks 123 that correspondto lines of glue on the peaks 17 of the corrugated sheet 13 as thecorrugated cardboard 12 is advanced past the probe or contact headassembly 66 during production in the apparatus shown in FIG. 1. Thus itcan be seen that the contact head assembly 66 is in effect measuring thedisturbance of the highly localized high frequency field by thecorrugated cardboard 12 as it passes underneath the contact head 66.This ability to sense the glue lines 18 is made possible because thehighly localized high frequency fields are distorted preferentially andvery strongly by any moisture that is present. Since the glue utilizedcontains moisture, there is a very large modification of the field asglue the passes under the contact head 66. By measuring the amplitude ofthe field detected, it is possible to obtain a very accurate estimate ofthe amount of water in the glue 18 at the joint. Since in the process ofmanufacturing the corrugated cardboard 12, the ratio of water in theglue is very tightly controlled, measuring the amount of water in theglue in contact with the top liner 16 at the peaks 17, it is possible todirectly ascertain the amount of glue that is in contact with this topliner 16 and carried by the peaks 17.

This measuring technique is particularly efficacious for use in themanufacture of double sided corrugated cardboard. In the process ofmanufacturing the corrugated cardboard it is possible to obtain goodadhesion between the peaks 17 on the bottom side of the corrugated sheet13 because the toothed roller 36 applies pressure to the joints betweenthe corrugated sheet 13 and the bottom liner 14. When the top liner 16is being placed, access cannot be had to the corrugated sheet 13 so thatthe present invention is very desirable for measuring the amount of glueon the peaks 17 which adhere to the upper liner 16.

By way of example in connection with the present invention, thereceiving coaxial line 62 would see approximately 1/1000 of a microwattof electrical energy when measuring the moisture content of very drypaper with no glue present on the far side. On the other hand, with aheavy glue line present on the far side, an output of 0.1 milliwattcould be achieved from the receiving coaxial line 62. In the graph shownin FIG. 9, the detector base line 121 appears at 0.2 of a volt whereasthe bottom portions 122 at approximately 0.4 volts of the waveform 106the top liner 16 and represents a measurement that is a combination ofthickness and moisture content of the liner. The peaks 123 which appearat approximately 0.8 volts have heights which represents the glue lineresponse and the variation in the glue lines which are encountered inthe corrugated cardboard at it passes under the contact head or probe66.

In connection with the present invention, the contact head 66 wasprovided with two very closely spaced coaxial lines 61 and 62 which havezones of sensitivity which can be generally represented as semi-circleswith the diameters of the semi-circles passing through the centers ofthe distal extremities 63 and 64 of the coaxial lines 61 and 62. Withthe coaxial lines having a diameter of suitable diameter as for example0.085", the center to center spacing between the two coaxial lines wouldbe approximately 0.085". Thus, the zone of sensitivity is also ahemisphere of approximately 0.085" in diameter. The electromagneticfield created at the distal extremity 63 of the coaxial line 61 isprimarily dominated by its electrostatic characteristics. The couplingbetween the distal extremities 63 and 64 of the coaxial lines 61 and 62in the absence of any added material is very weak. Typically powerlosses can be 1,000,000:1 up to 1,000,000,000:1. As explainedpreviously, the addition of any material in this field enhances thecoupling between the transmitting and receiving probes in the form ofthe distal extremity 63 and 64 of the coaxial lines 6I and 62. Theamount of enhancement is a function of the dielectric mass of a body ofmaterial that is in the vicinity of the distal extremities of thecoaxial lines 61 and 62 in which the distal extremities can beconsidered to be transmitting and receiving probes. As explained above,the amount of enhancement is a function of the content of any highdielectric constant components in the material. As for example, waterhas a very high dielectric constant on the order of 70 times that ofambient air. Paper may have a dielectric constant in the amount of 2-6times that of ambient air. Thus the apparatus and system hereinbeforedescribed will have moderate sensitivity to the presence of a dielectricsuch as paper and will be extremely sensitive to the presence of anymoisture in the vicinity of the coaxial electric fields which couple theprobes to each other. Although the probes or distal extremities 63 and64 of the lines 61 and 62 have been described as being in relativelyclose proximity to each other, it should be appreciated that somespacing between the coaxial lines can be permitted when that isnecessary. However, it is believed that the maximum spacing that shouldbe permitted between the distal extremities of the coaxial lines 61 and62 should not exceed 2-3 times the diameter of one of the coaxial tubes.

In selecting the diameter of the coaxial lines, it is desirable toselect the diameter so that the spacing between the centers of thecoaxial lines is less than the minimum resolution desired from thecontact head assembly. Thus, by was of example, if the flute pitch inthe corrugated cardboard 12 shown in FIG. 8 is 0.3", then the spacingbetween the centers of the two coaxial lines 61 and 62 should be lessthan this amount and preferably approximately one fourth of the flutepitch. In the case of the present invention a spacing of 0.085" isprovided which readily satisfies this requirement.

Although the present invention has been principally described inconnection with the sensing of moisture in corrugated cardboard toascertain whether there is adequate glue provided in the joints betweenthe upper peaks of the corrugated sheet and the upper liner, there areother numerous applications of the present invention. For example, suchmoisture sensing apparatus can be utilized for measuring the generalmoisture content of paper as it comes into a plant or as it is being fedthrough a paper mill. This can be accomplished by providing a hand heldinstrument incorporating the apparatus of the present invention whichcan be utilized for incoming quality inspection.

In another application, the sensor apparatus of the present inventioncan be utilized for drying sheet materials by controlling theapplication of heat to the material to remove excess moisture as forexample from paper in which the sensor apparatus of the presentinvention can be utilized to control the time that the paper is exposedto the heat to thereby control the removal of moisture from the sheetsof paper.

Other applications in which the invention can be utilized are measuringthe thickness of paint after it has been applied either to a metallicbackground or to a background that has a dielectric constantsignificantly different from that of paint. Such an apparatus and methodis particularly useful because it is non destructive of the paintedsurface.

Another application is measuring the thickness of egg shells after theyhave been layed which information can be utilized to control the balanceof the diet that is fed to the egg laying hens. Also the thicknesses ofwax surfaces on leaves of trees can be measured to ascertain the healthof the trees which for example can be correlated with the projectedproductivity of citrus from the trees. Medical applications include theability to differentiate between fatty tissue and non-fatty tissue closeto human skin.

In view of the foregoing, it can be seen that the moisture measurementapparatus, system and method utilizing microwave or high frequencyenergy has many applications making it possible to measure nondestructively the thicknesses of various bodies of materials havingsubstantially different dielectric constants. Since water has a highdielectric constant, some materials having water therein also have ahigh dielectric constant making such materials particularly suitable formeasurement in accordance with the present invention.

I claim:
 1. In apparatus for sensing the amount of glue being applied tothe peaks of a corrugated sheet to be disposed between upper and lowersheets to form cardboard having first and second substantially parallelsurfaces, a sensor device comprising first and second substantiallyparallel coaxial lines having distal extremities in relatively closeproximity to each other and adapted to be disposed in close proximity tothe cardboard solely on one side of the cardboard adjacent said firstsurface, transmitter means coupled to the first coaxial line forintroducing electrical energy in the high frequency to a microwave rangeinto the first coaxial line to cause a fringe radiation field to beestablished at the distal extremity of the first coaxial line and toextend into the cardboard, receiving means coupled to the second coaxialline for detecting any of the fringe radiation field coupled into itfrom the first coaxial line and measurement means coupled to thereceiving means for ascertaining when any change occurs in the couplingof the fringe radiation field from the first coaxial line to the secondcoaxial line.
 2. Apparatus as in claim 1 wherein the sensor deviceincludes a contact head adapted to be positioned in close proximity tosaid first surface and wherein the distal extremities of the coaxiallines are mounted in said contact head.
 3. Apparatus as in claim 2wherein said contact head has an arcuate surface and wherein the distalextremities of the first and second coaxial lines extend through saidarcuate surface.
 4. Apparatus as in claim 2 wherein said contact head isformed of an abrasion resistant material having high thermal andelectrical conductivity.
 5. Apparatus as in claim 4 wherein said contacthead is formed of material which has a thermal conductivity andelectrical conductivity which approximates that of aluminum. 6.Apparatus as in claim 5 wherein said contact head is formed of a siliconcarbide impregnated with aluminum.
 7. Apparatus as in claim 2 whereinsaid head is formed of Lanxide.
 8. Apparatus as in claim 2 together withspring means secured to said contact head for yieldably urging saidcontact head into engagement with said body of material with arelatively light force.
 9. Apparatus as in claim 8 wherein said springmeans is in the form of a spring arm secured to said contact head andmeans for adjusting the position of the contact head to vary thepressure with which the contact head is yieldably urged into engagementwith the body of material.
 10. Apparatus as in claim 8 wherein saidrelatively light force is in the order of 5 to 20 grams.
 11. In a systemfor measuring the moisture content of corrugated cardboard, first,second and third rolls of sheets of paper, means for advancing the sheetof paper from the first roll so that it serves as a bottom liner for thecorrugated cardboard, means for withdrawing the sheet of paper from thesecond roll and forming corrugations in the sheet of paper so that thesheet of paper has peaks disposed on opposite sides thereof, means forapplying glue to the peaks of the bottom side of the sheet of paper andcausing the peaks to come into contact with the sheet of paper from thefirst roll so that the corrugated sheet is adhered to the bottom liner,means for applying the glue to the peaks on the opposite side of thecorrugated sheet and means for withdrawing paper from the third roll ofpaper and bringing it into engagement with the peaks on the oppositeside having glue thereon to form a top liner, a contact head engagingthe top liner on the side opposite engaging the peaks of the corrugatedsheet, said contact head having first and second coaxial lines extendingtherethrough in a substantially parallel directions in close proximityto each other and engaging the top surface of the top liner, means forintroducing high frequency or microwave energy into the first coaxialline to cause a fringe radiation field to be created at the distalextremity of the first coaxial line and extending through the top liner,detector means coupled to the second coaxial line for detecting any ofthe fringe radiation field produced by the first coaxial line which iscoupled into the distal extremity of the second coaxial line andmeasurement means coupled to the detector means for ascertaining whenenhancement of the coupling of the fringe radiation field between thedistal extremities of the first and second coaxial lines occurs tothereby ascertain the presence of moisture in the glue securing the topliner to the corrugated sheet and means responsive to the sensedinformation for controlling the application to the glue of the peaks inthe upper side of the corrugated sheet.
 12. A system as in claim 11wherein the corrugated cardboard is moved relative to the contact head.13. A system as in claim 12 wherein the corrugated cardboard is moved ina direction which is perpendicular to the direction of the corrugationsin the corrugated cardboard.
 14. A system as in claim 11 together withmeans secured to the contact head for yieldably urging the contact headinto engagement with the upper surface of the top liner.
 15. A system asin claim 11 wherein the contact head is formed of an abrasion resistivematerial having high thermal and electrical conductivity.